Remove `IgnoreStandardErrorWarningFormat` from targeting pack generation

[ViktorHofer]

https://github.com/dotnet/source-build-reference-packages/blob/96f82a0db0bd0587e6523484b9b856a339c6cf30/src/targetPacks/Directory.Build.targets#L61

This setting hides real errors when generating assemblies via ILAsm. I noticed this when I accidentally removed this setting in the VMR. There are currently three errors that correspond to a single ILAsm bug which we should file an issue for so that everyone benefits.

  /__w/1/s/src/targetPacks/ILsrc/microsoft.aspnetcore.app.ref/6.0.2/ref/net6.0/Microsoft.AspNetCore.Mvc.Core.il(58668) : warning : Duplicate param name '_' in method '<CreateAsyncReleaser>b__2_0'
  /__w/1/s/src/targetPacks/ILsrc/microsoft.aspnetcore.app.ref/6.0.2/ref/net6.0/Microsoft.AspNetCore.Mvc.Core.il(58801) : warning : Duplicate param name '_' in method '<CreateAsyncControllerReleaser>b__2_0'
  /__w/1/s/artifacts/obj/Microsoft.AspNetCore.App.Ref.6.0.2/Release/netstandard2.0/ref/net6.0/Microsoft.AspNetCore.Components.patched.il(3478) : warning : Duplicate param name '_' in method '<CreateInitializer>b__7_0'

[mthalman]

Triage: It's not currently known the reason for setting IgnoreStandardErrorWarningFormat to true. Needs investigation.

[ViktorHofer]

Pretty sure it was because ilasm threw warnings back then that would have caused the assemblies to not get generated. We are still in that state. For the warnings above we should file an issue in runtime and get ilasm fixed.